Thermoplastically processible, substantially linear PVA/PU block copolymers, preferably polystyrene/polyurethane block copolymers could not be made available hitherto.
PVA/PUR mixtures are known. Thus, in accordance with GB 1,311,211 moisture-setting PUR prepolymers are mixed with polystyrene resins and the mixtures are used for sealing concrete surfaces. Expandable polystyrene beads can be processed together with PUR raw materials with heating to form a foam as described in JA 55 107 433 and JA 51 067 560.
To improve its impact resistance, thermoplastic PUR can be blended with from 40 to 60% of a maleic imide/styrene copolymer according to U.S. Pat. No. 4,287,314 and EP 41 347.
Some works such as Ruthkowska, Eisenberg, J. Appl. Polym. Sci. 29 (1984) No. 3, page 755; and 30 (1985) No. 5, page 3317 describe the miscibility of PUR with styrene/styrene sulphonic acid copolymers.
There are no fixed covalent bonds between the two components in any of these mixtures.
Fixed bonds between polyurethane and polystyrene segments do exist in the so-called interpenetrating polymer networks. Cross-linked PUR systems and cross-linked polystyrene systems are made up by simultaneous, independent polymer build up. Owing to the simultaneously interpenetrating networks, the polymers are joined together rigidly without covalent or ionic bonds existing among one another. The products can no longer be processed thermoplastically. (literature, for example: Frisch, Pure Appl. Chem. 43, (1975) page 229, Kaplan, Tschoegl, Polym. Eng. Sci., 15 (1975) page 343. Kim. Polym. Eng. Sci., 15 (1975), page 339, Kircher, Plast. Eng., 32(1976) page 32).
Compounds which have rigid covalent bonds between the PUR chains and polystyrene chains are obtained either by grafting of styrene on polyurethane with the aid of radical forming agents such as azodiisobutyronitrile (DOS 2,237,956, U.S. Pat. No. 4,299,924, DOS 3,223,396) or by radical copolymerization of unsaturated polyurethanes with styrene (U.S. Pat. No. 3,824,201, DOS 2,359,617, U.S. Pat. No. 4,521,546, DOS 2,704,424, Reinf. Plast. 28 (1984) No. 8, pages 209 to 211). These are radically cross-linked systems with markedly non-uniform molecular weight distribution, in particular of polystyrene segments. These products cannot be processed thermoplastically owing to the cross-linked structure and are therefore unsuitable for many applications, for example as coupling agents and/or polymer modifiers.
Telechelic styrene oligomers produced by radical polymerization with terminal amino and hydroxy groups have already been used for the production of polystyrene/polyurethane block copolymers. Okamoto (Int. Polym. Sci. Technol. 4 (1977), page 50) describes the addition of amino-terminated styrene oligomers during one-shot PU-synthesis, by which the mechanical properties are improved. Similar telechelatic polymers are described by Konter in Makrotmol. Chem. 182 (1981), page 2619. They are used as components in segmented and foamed PUR.
Polystyrenes produced by radical polymerization not only display a broad molecular weight distribution (M.sub.w .noteq.M.sub.n) but also lead to cross-linked end products, owing to the functional groups inside the chain which are undesirably formed during the necessary hydrogenation of the terminal nitrile group. There is therefore a risk that no thermoplastic block copolymers will be obtained if the functionality and therefore cross-linking are too high.